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Showing content from https://github.com/fmtlib/fmt/issues/4153 below:

I've hit a number of issues trying to package fmt for build2 with modules support.

A couple of I think recent regressions, addressed in this PR.
fmt.cc with import std; disabled will #include <expected> without any of the guards used for including it inside fmt/std.h, which raises a warning on MSVC. I haven't addressed this in the PR as FMT_HAS_INCLUDE isn't available at that point in the source file, and I'm also unsure why the C++ version and above macro are used in some places but feature test macros in others.
I guess the fmt modules support is not being verified by the CI? I'm also a bit unclear on what the status is of the tests wrt modules. I can see there is a module-test.cc, but before that gets added in the cmake file this check will bail out.

There appear to also be issues with the combination of modules and shared builds.

I've hit issues with the symbol export macros, however there are some unanswered questions here about what build2 should be doing, so it's not clear to me yet if anything will need to change on the fmt side for this particular problem.
Even if I work around the above, I end up with lots of linker errors about undefined symbols when building the tests against a shared modules build of the library (example output attached). I believe these derive from the fact that inside a module TU, member functions defined within the class definition are not implicitly inline as they are in traditional TUs. As such there are lots of functions that have not been marked as FMT_API (themselves or via their class) as they were implicitly inline; but now in the modules case they no longer are. There would appear to be two options: mark more classes/functions as FMT_API, or mark the member functions as explicitly inline. I wonder if the former can potentially cause problems for a non-modules build, but I'm generally unsure of the trade-offs here.

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